1. Field of the Invention
This invention relates to digital-to-analog decoders, or converters, and it relates, in particular, to such decoders which utilize time interpolation.
2. Description of the Prior Art
Many types of prior art digital-to-analog decoders employ very precise circuit elements in a single type of operation to convert input digital information into analog form with a given degree of resolution. By contrast, however, time interpolation decoders achieve similar resolution by making at least one coarse determination of what a piece of information should be; and they then change that determination in various ways during a given time interval in order to obtain an average analog signal amplitude which equals the value of the input digital information. The philosophy of the time interpolation decoders is to take advantage of the fact that digital techniques allow a fine determination of the time of operation of a circuit with relatively noncritical circuit elements, as compared to utilizing analog techniques entirely in order to make a fine determination of analog signal amplitude. There have been few time interpolation digital-to-analog decoders taught in the prior art. Three examples of such decoders are here noted and all are copending U.S.A. patent applications assigned to the same assignee as the present application.
A first example is the J. C. Candy application Ser. No. 412,296, filed Nov. 2, 1973, now patent No. 3,893,102, and entitled "Digital-to-Analog Converter Using Differently Decoded Bit Groups." In it Candy teaches a digital-to-analog converter illustratively employing a 2-level technique and a single amplitude step from the initial coarse analog value to one higher amplitude at a digitally determined time for establishing the overall analog average value.
A second time interpolation decoder example is that in the copending application of J. C. Candy, S. L. Freeny, and W. H. Ninke, Ser. No. 597,087, filed July 18, 1975, and which is a continuation of their application Ser. No. 421,678, filed Dec. 4, 1973 entitled "Digital-to-Analog Converter with Digitally Distributed Amplitude Supplement" and now abandoned. In that application, a 2-level decoding technique is again employed; but multiple amplitude steps between the aforementioned coarse initial analog value and the one other analog value are employed during a character time to establish the desired average analog amplitude.
The third time interpolation decoder example is included in an R. C. Brainard and J. C. Candy application Ser. No. 461,878, filed Apr. 18, 1974, now Pat. No. 3,925,731, and entitled "Differential Pulse Coded System Using Shift Register Companding." That application teaches a digital-to-analog converter in which the analog signal is derived from a difference pulse coded bit stream. Multiple discrete analog signal levels are obtained from a running digital accumulation operation to produce a stepped analog signal which has the desired average value over a Nyquist period of the underlying analog information represented by the digital input to the converter.
One problem with the prior art digital-to-analog converters for time interpolation systems is that commercial transmission of digital signals between distant points is usually achieved by pulse code modulation (PCM) signals which are formed in accordance with either a linear or a companded coding rule. Thus, bit series decoders of the type in the Brainard et al, decoder cannot be conveniently utilized in commercial systems unless they are preceded by a circuit for restoring the difference pulse coded format. However, such additional code translation circuits usually respond to the full PCM word and, thus, necessarily increase the cost of the decoding operation to a substantial extent. The other decoder examples hereinbefore mentioned work from a PCM digital signal word, but they do not employ the shift register type of accumulation utilized in the coder of the type taught in the aforementioned Brainard et al. application for cooperating with the decoder which is also there taught. That Brainard et al. coder is the only one currently known in the art to use time interpolation as hereinbefore described. Thus, if the decoders of the first and second examples mentioned are employed to operate on a PCM signal format, it is necessary to include an inventory of one type of equipment for coders and a different type of equipment for decoders since, except for resistor networks, the equipment employed is not generally common to the coder and decoder.